None.
Not Applicable.
1. Field of the Invention
The present invention relates to a thermal ink jet recording apparatus employed for recording information in the form of visual images and symbolic characters by means of electrically effecting the ejection of ink droplets onto an ink receiving/recording media (e.g. sheets of paper and the like). More particularly, the present invention relates to a method and apparatus for detection of potentially damaging overcurrent situations in an ink jet print head.
2. Related Art
Ink jet recording apparatus have several well known advantages. For example, the noise level generated by printing/recording is so low as to be negligible and ordinary sheets of paper may be employed without processing and/or coating special synthetic materials on the surfaces thereof. There exist various kinds of ink jet ejecting methods used in the ink jet recording apparatus and in recent years, some of these methods have been put into practical uses.
Among the various kinds of ink jet ejecting methods, one ink jet ejecting method that has proved not only viable, but reliable and relatively inexpensive is described in U.S. Pat. No. 5,319,389, issued on Jun. 7, 1994 to Ikeda et al. Described in that patent is an ink jet ejecting method which employs kinetic energy for ejecting ink droplets by transferring thermal energy into the ink. In this method, thermal energy from a liquid-to-vapor transition of the ink leads to a rapid volumetric change in the ink. An ink droplet is ejected from an ejection outlet formed at the front of a recording head. The ink-receiving or ink-recording medium is placed close to the nozzle, and the ejected droplet reaches the surface of the recording medium, thus establishing printing.
A print head used in the above described ink ejecting method, in general, has an ink ejection outlet for ejecting ink droplets, and an ink liquid passage which communicates with the ink ejection outlet. The ink liquid passage includes an electro-thermal converting element for generating the thermal energy. The electro-thermal converting element includes a resistance layer for heating by applying a voltage between two electrodes in the material. In this kind of a print head, forces are applied into the ink in the ink liquid passage, which are induced by capillary action, pressure drops or the like, and are balanced so that a meniscus is formed in the liquid passage adjacent the ink ejection outlet. Every time an ink droplet is ejected, by means of the balanced forces applied to the ink, ink is drawn into the ink passage and a meniscus is formed again in the ink passage adjacent the ink ejection outlet.
The other type of ink jet printer utilizes a piezo-electric crystal element instead of the thermal element. The crystal expands when energized causing the ink to be sprayed from the nozzle.
There are numerous difficulties that may occur with an ink jet system such as that heretofore described. For example, the active nozzle heater driver circuit, including the heater, for applying thermal energy to the ink, is often located on an integrated circuit chip (as opposed to discrete components). The active nozzle heater circuits (if field effect transistors are used) normally have their sources connected to ground on the chip. The ground is conventionally wired through the chip, and small bits of contamination may cause a low impedance short or an actual short. Many times in the manufacture of such integrated chips, a layer associated with the heater resistor may be inadvertently connected to ground or punched through for connection to another resistance layer. The increased current through the external line driver results in breakdown or failure of the driver after prolonged operation. Moreover, during connection to the pads of the chips to the external electronic circuitry of the machine, occasionally the TAB bonder machine errs and connects the ground beam to the data line pad on the heater on the chip, causing a data line to ground short circuit. (This kind of short also may occur with address lines.) The result of any of these type manufacturing errors, of course, may result in xe2x80x9cblownxe2x80x9d line drivers.
Conventionally the interconnection between the chip and the external world is through a TAB circuit or tape that connects the data line to the heater chip pads and another pad to ground. The tape or TAB circuitry is coated to inhibit ink that happens to spread under the TAB circuit, from shorting lines on the circuit. Occasionally this coating may be flawed and may include voids. Moreover, ink deposited in a manner to underlie (partially) a TAB circuit, tends to migrate or grow over time between the ground TAB circuit and the data TAB circuit. This occurs because the ink is ionic, and the positive and ground potential will tend to be attractive to the ink. Once a bridge-like contact occurs, a short condition exists and line driver destruction is likely to occur.
While such manufacturing caused defects and shorted conditions should be detected in the chip electrical test, the faults described may be intermittent or occur only after a period of operation (e.g. the ink migration condition mentioned above). Moreover, the chip electrical test acts as a bottleneck to increased production. Therefore, it is advantageous, as will be seen hereinafter, to allow for dynamic testing under usage conditions, which will permit testing in the machine in a manner to inhibit catastrophic breakdowns, especially with respect to driver circuits.
Failure to detect a short circuit in the print head can cause damage to the voltage regulator, the print head itself or in some cases damage the entire printer. The result of an undetected short can range from poor print quality to necessity of replacing the entire printer.
In the ink jet printer art, testing of ink jet printers to protect against short circuits due to ink contamination of high voltage electrostatic plates, is well known. For example, in U.S. Pat. No. 4,171,527 a circuit is disclosed which senses the fouling of an electrostatic ink jet head and causes shutoff of the head and of the associated electronics. Ink fouling is sensed by detecting contamination of the charge electrodes or of the deflection plates by conductive ink. The circuit employs a strobe in conjunction with a comparator which acts as a gate so that testing occurs only upon command.
U.S. Pat. No. 4,119,973, issued on Oct. 18, 1978 discloses a fault detection and compensation circuit for ink jet printer wherein the control circuitry monitors the potential of the deflection electrode and if an electrode short substantially persists for a period of time greater than a preselected period, the printer will be disabled and the printing operations will be terminated. See FIGS. 1-4, column 2 lines 10-45 and claims 1-4. Again, the patent deals specifically with highly conductive ink, and electrostatic ink jet printing.
U.S. Pat. No. 4,439,776, issued on Mar. 27, 1984, discloses ink jet charge electrode protection circuitry wherein the operational status of each charge electrode is determined by monitoring either the voltage level of the electrode or the current flowing to the electrode. If the voltage level is below a defined level or the current flow is above a defined level, a fault condition is detected and the charge electrode supply voltage of the ink jet printer is shut down to avoid damage, specifically to the charge electrodes.
U.S. Pat. No. 4,774,526 discloses a method of measuring the current flowing into the print head in order to detect a short. This method requires the printer to be in a stable state (i.e. nothing in operation) prior to measuring for a fault. Current is applied through the test circuit and measured after traveling through the print head in order to determine if a short exists causing a drop in the expected measured current.
U.S. Pat. No. 5,736,997 discloses a method of measuring the voltage to detect a lower than expected impedance. This patent also discloses a method of inhibiting activation of the region suspected of containing a short.
The advent of the Multi-Function Printer (MFP) or All-in-One (AIO) device has necessitated a change in the overcurrent protection scheme employed to protect these devices. Those systems utilizing a current measurement scheme require an idle state to be effective. This affects throughput as the scanner motor or scanner lamp must be idled or shut off prior to testing. Other systems utilizing a voltage measurement scheme also require the above idle state to be effective.
There is a need for an overcurrent detection scheme for the printer which can be employed at any time. Specifically, an overcurrent detection scheme is needed which can test for the presence of a short circuit between swaths of a print head that does not require the rest of the machine to be idled. A calibration process is needed to account for manufacturing tolerances in order to reduce the incidence of a false positive overcurrent detection. Finally, a re-calibration process is needed to account for the age related degradation of a system and to allow re-testing to confirm an overcurrent detection.
The instant invention meets all of these needs. The invention is drawn to a test circuit which measures the voltage decay over time. Specifically, the test circuit utilizes a known resistor connected to the power source (voltage) and the printer. The printer functions as a capacitor to complete the test circuit as an RC circuit. After isolating the power source from the test circuit, the voltage decay is monitored to indicate the presence of a short circuit. It is well known that the voltage of an RC circuit will decay to a third of the initial voltage over a period of time called the Time Constant (TC). In use, the test circuit measures the voltage prior turning the voltage off, and then at a time equal to the TC. If the subsequently measured voltage is less than one third of the initially measured voltage then a short circuit may exist.
Initially contemplated for the invention was a test circuit in which the TC was computed from specified system components. Due to the wide range of manufacturing tolerances for the system components, it became likely that a short circuit would be detected that did not exist (false positive) or a short circuit would not be detected that actually exists (true negative). Accordingly, a calibration method was developed using the test circuit of the invention to determine the actual TC of the system. As a result, the overcurrent detection scheme becomes specific to the machine on which it is implemented. Because the invention accounts for manufacturing tolerances, manufacturing costs can be lowered by allowing use of cheaper components (having a greater range of acceptable capacitance).
Finally, it was discovered that age related degradation of the printer capacitance could cause the false detection of a short circuit on an older machine. The invention allows the test circuit to be re-calibrated to account for this age related change. Thus, if a short circuit is detected, the operator has the option of recalibrating the system and re-testing to confirm the fault. As a result, the useful life of the printer can be extended.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.